1. Field of the Invention
This invention relates to an infrared-detecting element using a piezoelectric film and to an infrared image sensor using the same.
2. Background Art
All of infrared-detecting elements used generally for consumers are uncooled-type infrared-detecting elements and classified roughly to pyroelectric type and bolometer type.
In a pyroelectric-type infrared sensor, it is necessary that a special material called as pyroelectric body is used as the infrared-detecting element. Representatively, a material of a lead titanate or a barium titanate, which are ferroelectric materials of oxide, is used. However, in these materials, crystallization temperature is high, and ion etching is difficult, and a precious-metal electrode material is required because the materials are oxides. From such reasons, there is a problem that it is difficult to prepare the materials directly on a silicon substrate for an infrared sensor produced by CMOS process.
Accordingly, instead of the monolithic process of CMOS, a complicated expensive hybrid process using a bonding technique or the like cannot help being used.
For example, as an example of the pyroelectric-type infrared-detecting element, there is a method for producing the infrared-detecting element, including: forming an orientational oxide ground film on a substrate; forming the pyroelectric body and a resin film thereon; then bonding another substrate thereto; and then dissolving and removing the orientational oxide ground film.
However, the pyroelectric-type infrared-detecting element has had some problems.
The first problem is that the complicated steps of bonding and of dissolving and removing the substrate in the production of the detecting element become large obstacles in reliability, accuracy, microfabrication, and preparation of array.
The second problem is that an amplifier connected to the infrared-detecting element cannot help being provided on a different substrate from the substrate of the infrared-detecting element, and the infrared-detecting element and the amplifier come to be connected with interconnection. Therefore, in comparison with the case that the infrared-detecting portion and the amplifier are formed in a lump on a single substrate only by a thin-film step, noise or the like easily enters therein and the detection accuracy deteriorates.
On the other hand, in a bolometer-type infrared sensor, temperature coefficient of resistance when infrared is irradiated to a resistance element is utilized and detected. As the resistance element, a thermistor, a p-n diode, or the like can be used, and can be monolithically produced on a silicon substrate, and therefore, has been actively researched. However, the largest problem of the bolometer-type infrared sensor is badness of detection sensitivity. For reading the resistance value, it is necessary that bias current is flowed through the resistance element and therefore the change amount of generated voltage due to temperature is detected. However, with respect to various noises such as Johnson noise contained in voltage generated in the resistance element, the voltage change amount generated by temperature change by infrared irradiation is small and therefore S/N ratio is bad and detection with good sensitivity is very difficult.
Moreover, instead of the bolometer type and the pyroelectric type, there has been proposed the infrared-detecting element of pseudo pyroelectric effect type by which temperature rise by infrared irradiation is detected through thermal strain by using a piezoelectric body as the detection element (Japanese Patent No. 3311869).
In the Japanese Patent No. 3311869, the piezoelectric film includes compound semiconductors such as PbTe, PbSe, PbS, HgTe, HgSe, Hg1-xCdxTe, GaSb, GaAs, InP, InAs, InSb, Ge, Mg2Si, Mg2Ge, Mg2Sn, Ca2Sn, Ca2Pb, ZnSb, ZnAs2, Zn3As2, CdSb, CdAs2, Cd3As2, Bi2Se3, Bi2Te3, Sb2Te3, As2Se3, As2Te3, PtSb2, In2Se3, and In2Te3. However, when such a material is film-formed on a silicon substrate, it is not easy to produce a single crystal film having sufficient piezoelectricity or a high orientational film. And, a particular devisal therefor has not been disclosed, either. Moreover, the particular desirable piezoelectric film includes HgTe, Hg1-xCdxTe, InSb, Cd3As2, Bi2Te3, and PtSb2, which are narrow band gap semiconductors. However, because the band gap is narrow, leak current is necessarily very large and it involves very large difficulty to perform reading in room temperature. Moreover, thermal expansion difference between the piezoelectric body and the substrate is utilized and the piezoelectric body is formed directly on the substrate, and therefore, the received heat by infrared irradiation is easily diffused in the substrate, and the temperature of the piezoelectric body does not rise and the obtained electromotive force is slight.
As described above, every one of pyroelectric-type, bolometer-type, and pseudo-pyroelectric-effect-type infrared-detecting elements and infrared image sensors using the elements, which are conventional techniques, has problems. With respect to the problems, many attempts have been made. However, every one of the attempts has both merits and demerits, and therefore, in the present situation, the above-described techniques has not been substituted.